1. Field of the Invention
This invention relates to moving coil type electromagnetically driven shutters and more particularly to the prevention of bouncing of the shutter blade drive rotor.
2. Description of the Prior Art
In general, the moving coil type electromagnetically driven shutters open the shutter blades by applying an electromagnetic force to the drive rotor for the shutter blades. During a long exposure, the application of the electromagnetic force continues in the exposure aperture fully open position. Then, when the shutter blades are closed, the electromagnetic force is no longer applied to allow for the reaction of the return spring.
As the shutter blades approach the fully open position, the reaction force of the retrun spring is increased. It is therefore required that the electromagnetic drive force for opening the shutter blades be of magnitude above what is necessary. The use of such excessive electromagnetic force in driving the rotor produces a disadvantage in that when the rotor is stopped by a stopper at the position where the shutter blades are fully open, the rotor is apt to bounce. Thus, when the rotor arrives at the stopper, a shock is produced which impels the shutter blades to move in the closing down direction. As the reverse drive force is balanced with the electromagnetic force, the shutter blades move forward again. Such a procedure repeats itself in a very short period of time against the stopper. Assuming that a shutter closing signal appears at a time during bouncing of the shutter blades, then the net exposure time is erroneously elongated or shortened depending upon the bouncing position of the shutter blades. That is, in case the deactuating signal occurs when the shutter drive rotor is bouncing in a direction to open the shutter blades, the time lag from the moment at which the shutter deactuating signal has appeared to the moment at which the shutter starts to close is relatively long. On the other hand, when the bouncing of the rotor in a direction to close the shutter blade coincides with the appearance of the deactuating signal, the above defined time lag is relatively short. Thus, the occurrence of the bouncing of the rotor at the start of closing of the shutter blades leads to deviation of the exposure value by a considerable amount.
Aside from this drawback, it is desirable to make the shutter drive rotor as light in weight as possible. Therefore synthetic resin has found use as the material of the rotor. As the shutter drive rotor is struck against the stopper repeatedly in one cycle of shutter operation, it is however found that the strength of the rotor is insufficient in the active area thereof for contact with the stopper. After a small number of cycles of shutter operation, it will be seen that the rotor is deformed or cracked.